Described below is a machine system    a) having a rotor which can be rotated about a rotation axis, is enclosed by a stator and has at least one rotor winding whose superconductive conductors are thermally conductively coupled to a central cylindrical rotor cavity extending in the axial direction,    b) having a thermally conductive body, which has a central refrigerant space and projects statically into the rotor cavity while maintaining an annular gap,    c) having a thermal contact gas located in the annular gap,    d) having a stationary refrigeration unit which is located outside the rotor and includes a condenser space    and    e) having tubular line sections extending between the central refrigerant space of the thermally conductive body and the condenser space of the refrigeration unit.
The central refrigerant space, the tubular line sections and the condenser space in this case form a closed line system, in which a refrigerant is circulated or can be circulated by utilizing the thermosiphon effect.
A corresponding machine system is disclosed by WO 02/15370 A1.
Metal oxide superconductor materials with critical temperatures Tc above 77 K have been known since 1987. These materials are therefore also referred to as high-Tc superconductor materials or HTS materials, and in principle they permit a cooling technique using liquid nitrogen (LN2).
Attempts are also being made to produce superconducting windings of machines with conductors which employ such HTS materials.
It has, however, been found that previously known HTS conductors have a comparatively low current-carrying capacity in magnetic fields with inductions in the tesla range. Despite the high critical temperature Tc per se of the materials being used, this often means that the conductors of such windings must still be kept at a temperature level lying below 77 K, for example between 10 and 50 K, so that they can carry significant currents when high field strengths occur. Such a temperature level is much higher than 4.2 K, the boiling temperature of liquid helium (LHe) with which known metallic superconductor materials with a comparatively low critical temperature Tc, so-called low-Tc materials or LTS materials, are cooled.
Refrigerating systems in the form of so-called cryo-refrigerators, with a closed pressurized He gas circuit, are preferably used to cool windings of HTS conductors in a temperature range below 77 K. Such cryo-refrigerators are in particular of the Gifford-McMahon or Stirling type, or are designed as so-called pulse-tube refrigerators. They also have the advantage that they are refrigerating power is available virtually at the touch of a button, and the handling of low-temperature liquids is avoided. When using such refrigerating systems, the superconducting winding is merely cooled indirectly by thermal conduction to a cold head of a corresponding refrigerator (cf. also for example “Proc. 16th Cryog. Engineering Conf. (ICEC 16)”, Kitakyushu, J P, 20-24 May 1996, Elsevier Science Publishers, 1997, pages 1109 to 1129).
A corresponding refrigeration technique is also provided for the rotor of an electrical machine as disclosed by WO 02/15370 A1 mentioned in the introduction. The rotor contains a rotating winding of HTS conductors, which are located in a thermally conductively designed winding carrier. This winding carrier is equipped with a cylindrical rotor cavity extending in the axial direction. A central thermally conductive body, which encloses a central cylindrical refrigerant space, projects statically into this rotor cavity. In the hollow cylindrical annular gap formed between the co-rotating outer wall of the rotor cavity and the stationary outer wall of the thermally conductive body, there is a thermal contact gas for heat transfer between the winding carrier and the thermally conductive body. Stationary tubular line sections, extending laterally out of the rotor, connect with its central refrigerant space. These line sections lead into a refrigeration unit's condenser space lying geodetically higher, and they form a closed single-tube line system with this condenser space and the central refrigerant space. This line system contains a refrigerant, which circulates by utilizing a so called thermosiphon effect. Refrigerant condensed in the condenser space is conveyed by the tubular line sections into the central refrigerant space, where it absorbs heat because of the thermal coupling to the winding carrier via the thermal contact gas and therefore to the HTS winding to be cooled, and is at least partially evaporated. The evaporated part of the refrigerant then travels back through the same line sections into the condenser space, where it is re-condensed. The refrigerating power required for this is provided by a refrigerating machine, the cold head of which is thermally coupled to the condenser space. The return flow of the refrigerant to the refrigerating machine's parts acting as a condenser is driven by a slight positive pressure, which is formed in the central refrigerant space acting as an evaporator part. This positive pressure generated by the creation of gas in the evaporator part and the liquefaction in the condenser space leads to the desired return flow of refrigerant. The corresponding circulation is also referred to as natural convection.
Instead of this known single-tube thermosiphon line system in which the liquid refrigerant and the gaseous refrigerant flow through the same tube sections, double-tube line systems are also known for refrigerant recirculation by utilizing a thermosiphon effect (cf. for example WO 00/13296 A). In this case, an additional tube for the gaseous refrigerant must be provided in the region of the hollow shaft of the rotor.
In the known machines with thermosiphon cooling, the refrigerant is thus transported merely by utilizing natural convection so that no other pump systems are necessary. If such a machine system is intended to be used on ships or offshore installations, then it is often necessary to deal with static trims of up to ±5° and/or dynamic trims of up to ±7.5° in the longitudinal direction. In order to receive approval from a classification society for use on ships, the cooling system of such a machine system must consequently ensure reliable cooling even under these conditions. If trims of the machine are intended to be tolerated, however, the risk arises that a region of the tubular line parts between the central refrigerant space and the refrigeration unit will come to lie geodetically lower than the central refrigerant space. The effect of this would be that, under the effect of gravity, the refrigerant can no longer reach the refrigerant space. Cooling of the machine and thus operation thereof would therefore no longer be ensured.
In order to counter this risk, the following proposals inter alia are known:    One simple solution might consist in arranging the machine inclined relative to the horizontal so that, in the thermosiphon line system, there is still a gradient in the direction of the central refrigerant space even in the event of the greatest expected trim or oscillation amplitude. A correspondingly inclined arrangement is undesirable particularly in the maritime sector, especially with sizeable machine lengths, because of the large space requirement then entailed.    In principle, the refrigerant may also be forcibly circulated by a pump system. Considerable equipment outlay is necessary for this, however, especially when the refrigerant is intended to be at a temperature level of for example 25 to 30 K. Such circulation systems furthermore cause significant heat losses, and can scarcely fulfill the service life requirements of the maritime sector with its long maintenance intervals.